Pain Management

Definitions and Physiology

Definitions

• Nociception: process through which peripheral receptors transmit information about current (or potential) tissue damage centrally as pain
• Nociceptor: receptor in end-organ that detects biochemical changes associated with current or potential tissue damage
• Nociceptive pain: pain caused by actual or threatened damage to non-neural tissue
• Neuropathic pain: pain caused by pathology in the somatosensory nervous system
• Central sensitization: increased responsiveness of nociceptive neurons in the central nervous system to normal or subthreshold input
  • Allodynia: pain due to a stimulus that does not typically provoke pain
  • Hyperalgesia/hyperpathia: heightened response to a typically painful stimuli

Physiology

• Complex bidirectional process with phases: transduction → transmission → modulation → central perception
• Peripheral nerves, both motor and sensory, are grouped by size and myelination. 
• Pain is experienced in 2 phases:
  1. First phase is mediated by the fast-conducting A-delta fibers, associated with an initial extremely sharp pain.
  2. Second phase is mediated by C fibers, associated with a more prolonged and a less intense feeling of pain.
• Type-A fibers: large and myelinated, thus fast conducting
  • A alpha:
    • Primary receptors of the muscle spindle and Golgi tendon organ
  • A beta: 
    • Largest-diameter afferent axon
    • Secondary receptors of the muscle spindle, contribute to cutaneous mechanoreceptors
    • Perceive light touch and/or moving stimuli
  • A delta: 
    • Free nerve endings that conduct stimuli related to pressure and temperature
    • Action potential conducts at a rate of approximately 20 m/s towards the central nervous system (CNS)
  • A gamma: 
    • Motor neurons that control the intrinsic activation of the muscle spindle
• Type-B fibers:
  • Middle-sized, thinly myelinated fibers
  • Responsible for autonomic information
• Type-C fibers:
  • Unmyelinated nociceptor slow fibers (conduct at a rate of approximately 2 m/s)
  • Respond to combinations of thermal, mechanical, and chemical stimuli

Types of Pain

Management of Pain

General principles

• Must be tailored to each patient’s circumstances, perspective, and physiologic condition
• Requires a systematic assessment and regular reassessments:
  • Type: throbbing, cramping, burning, stabbing, etc.
  • Periodicity: continuous, with or without exacerbations or incident
  • Location
  • Intensity (may be determined with a visual analog scale)
  • Modifying factors
  • Effects of treatments
  • Functional impact
  • Impact on patient
• Whenever possible, use targeted, disease-specific treatment.
• Incorporate non-pharmacologic adjuncts and maximize the use of non-opioid analgesics before the use of opioids. 
• If an opioid is prescribed for pain:
  • Use short-acting agents only
  • Use for the shortest duration possible 
  • Screen for risk of opiate misuse
  • Utilize local prescription monitoring program
  • Counsel patients on safe storage and disposal

Management of chronic pain

The following principles are recommended by the World Health Organization (WHO) as a basis for the treatment of chronic pain:

• “By the clock”: Analgesics should be given at regular intervals. The frequency depends on whether it is a long- or short-acting preparation. 
• “By the mouth”: If possible, drugs should be administered orally. If the oral route is not feasible, the least invasive route should be considered (e.g., sublingual or subcutaneous before IV). 
• “By the ladder”: Stick to the 3-step system (see figure below). Drug selection should be appropriate to the severity of the pain. With severe pain, it may be appropriate to begin at the top of the ladder with a strong opioid. It is usually not necessary to step down unless the cause of pain is believed to have resolved. 
• “By the individual with attention to details”: Dosing of pain medication should be adapted to the individual, as every patient responds differently. To optimize adherence and outcomes, the patient and those who care for them should be provided with a written program.

Most commonly used substances

• Non-opioid medications
  • Nonsteroidal anti-inflammatory drugs (NSAIDs) (ibuprofen, naproxen)
  • Acetaminophen
  • Aspirin
• Weak opioids
  • Codeine 
  • Tilidine
  • Dihydrocodeine 
  • Tramadol (depending on the dose, tramadol can also behave as a strong opioid)
• Strong opioids 
  • Pethidine 
  • Morphine 
  • Oxycodone 
  • Methadone
  • Hydromorphone 
  • Buprenorphine
  • Fentanyl 
• Adjuvant analgesics (for specific indications)
  • Alpha-2 adrenergic agonists (e.g., clonidine, tizanidine): support effect of opioids, allow for reduction of opioid dosages in acute postoperative pain, chronic pain, neuropathic pain
  • Anticonvulsants (e.g., gabapentin, phenytoin, carbamazepine, pregabalin): e.g., neuropathic pain (especially trigeminal neuralgia)
  • Antidepressants (tricyclics, serotonin-norepinephrine reuptake inhibitors (SNRIs)): e.g., neuropathic pain, cancer pain, migraine, tension headache, postherpetic neuralgia
  • Beta-blockers (metopropolol, propanolol, timolol): prevention of migraine headaches
  • Bisphosphonates: e.g., cancer-related bone pain, osteoarthritis
  • Botulinum toxin: refractory chronic pain, especially trigeminal neuralgia, postherpetic neuralgia, migraine
  • Cannabis and cannabinoids (e.g., nabiximols, dronabinol, nabilone): e.g., cancer pain
  • Corticosteroids: e.g., neuropathic pain, bone pain, headache caused by increased intracranial pressure 
  • N-methyl-D-aspartate (NMDA) receptor antagonists (ketamine): e.g., acute pain, perioperative pain
  • Topical agents (e.g., lidocaine, capsaicin, diclofenac patch, ketamine cream, gabapentin cream): regional neuropathic pain

Approach to specific types of pain